No single component is more important in the manufacture of hot mix asphalt than the aggregate dryer and its exhaust system. One problem encountered with the use of such apparatus is pollution in the form of NO.sub.x compounds produced by the burner flame. It is known that the formation of NO.sub.x compounds may be inhibited by more efficient combustion of the available fuel; reducing the amount of nitrogen in the fuel; reducing the flame temperature; reducing the amount of air available for combustion; and reducing the time that combustion gases spend at elevated temperatures.
It is common in the steam generation industry to lower flame temperature by recirculating flue gas to the burner and thereby reducing NO.sub.x emissions. This reduction in flame temperature is further augmented by staged combustion in which the flame is initially oxygen poor (and therefore cooler) and is charged with additional oxygen a short time later to complete combustion. Multiple stages are preferably utilized to obtain the best results.
Experience has taught, however, that methods useful in the steam industry for reducing the formation of NO.sub.x compounds are not applicable to equipment used in the production of asphaltic products, such as aggregate dryers. This is because the two processes utilize different types of flames to provide heat and because aggregate dryers generally are of a shorter dimension unsuitable for implementing staged combustion techniques having multiple stages. Steam generation plants typically utilize lengthy staged combustion and a flame characterized as long and lazy. Lengthy, multiple staged combustion set-ups and long, lazy flames cannot be used in aggregate dryers because aggregate dryers typically provide a smaller combustion area than do steam plants.
The recirculation of gases in rotary heating equipment for purposes other than to reduce the level of NO.sub.x is known in the art. U.S. Pat. No. 4,190,370 discloses a drum mixer having a temperature control system for regulating the temperature of the asphalt-aggregate mix by varying the flow of hot gases through the drum mixer. The system is also disclosed in connection with an aggregate dryer. The temperature control system withdraws gases exiting the drum before they pass through a baghouse and recirculates them to an input manifold on the drum mixer. This recirculation system reduces the temperature of the burner flame and the energy required to heat the gases within the drum mixer, but does not suggest any effect on NO.sub.x emissions.
U.S. Reissue Pat. No. Re. 29,496 discloses another rotary heating device in which combustion gases are recirculated from the outlet of a drum mixer to a burner assembly located at the inlet of the drum mixer. The recirculation gases are passed through a heating or a cooling heat exchanger before being routed to the burner. This recirculation scheme is said to provide a somewhat isothermal air flow to the burner and to allow more energy efficient operation, but the patent does not discuss any reduction in either flame temperature or flame length. Nor does the patent suggest that the scheme operates to reduce NO.sub.x emissions.
Other examples of rotary heating devices incorporating various gas recirculating schemes are disclosed in U.S. Pat. Nos. 3,963,416; 4,143,972; 4,309,113; 4,332,478; 4,600,379; and 4,892,411. However, none of these recirculation methods are directed to the reduction of NO.sub.x emissions.
Therefore, there remains a need for an improved rotary heating device for use in the production of asphaltic paving materials having reduced NO.sub.x emissions, and in particular for a combustion chamber which consumes fuel in a manner which results in fewer NO.sub.x emissions.